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在网络时代,XML文件起到了一个保存和传输数据的作用。Soap协议通过Xml交流信息，数据库通过Xml文件存取等等。那么怎样快速的从一个XML文件中取得所需的信息呢？
我们知道，JAVA的JAXP中和Microsoft.Net都有Xml分析器，Microsoft.Net是边读边分析，而JAXP是读到内存中然后才进行分析（还有一种是事件机制去读），总而言之，是不利于快速读取。基于此，Microsoft.Net 和JAXP都提供了XPATH机制，来快速定位到XML文件中所需的节点。
例如有一个XML文件：booksort.xml:
？xml version="1.0"？
!-- a fragment of a book store inventory database --
bookstore xmlns:bk="urn:samples"
book genre="novel" publicationdate="1997" bk:ISBN="1-861001-57-8"
titlePride And Prejudice/title
author
first-nameJane/first-name
last-nameAusten/last-name
/author
price24.95/price
/book
book genre="novel" publicationdate="1992" bk:ISBN="1-861002-30-1"
titleThe Handmaid's Tale/title
author
first-nameMargaret/first-name
last-nameAtwood/last-name
/author
price29.95/price
/book
book genre="novel" publicationdate="1991" bk:ISBN="1-861001-57-6"
titleEmma/title
author
first-nameJane/first-name
last-nameAusten/last-name
/author
price19.95/price
/book
book genre="novel" publicationdate="1982" bk:ISBN="1-861001-45-3"
titleSense and Sensibility/title
author
first-nameJane/first-name
last-nameAusten/last-name
/author
price19.95/price
/book
/bookstore
如果我们想快速查找”last-name”等于”Austen”的所有标题名，可以通过以下方法可以得到：
XmlReaderSample.cs
//Corelib.net/System.Xml.Xsl/XPathDocument Class
//Author :Any
using System;
using System.IO;
using System.Xml;
using System.Xml.XPath;
public class XmlReaderSample
{
public static void Main()
{
XmlTextReader myxtreader = new XmlTextReader("booksort.xml");
XmlReader myxreader = myxtreader;
XPathDocument doc = new XPathDocument(myxreader);
XPathNavigator nav = doc.CreateNavigator();
XPathExpression expr;
expr = nav.Compile("descendant::book[author/last-name='Austen']");
//expr.AddSort("title", XmlSortOrder.Ascending, XmlCaseOrder.None, "", XmlDataType.Text);
XPathNodeIterator iterator = nav.Select(expr);
while (iterator.MoveNext())
{
XPathNavigator nav2 = iterator.Current;
nav2.MoveToFirstChild();
Console.WriteLine("Book title: {0}", nav2.Value);
}
}
}
运行这个程序，结果为：
Book title: Pride And Prejudice
Book title: Emma
Book title: Sense and Sensibility
可以看到查找正确。
利用XPATH中的一些功能，也可以实现简单的排序和简单运算。如在数据库中经常要对数据进行汇总，就可用XPATH实现。
如：
order.xml
!--Represents a customer order--
order
book ISBN='10-861003-324'
titleThe Handmaid's Tale/title
price19.95/price
/book
cd ISBN='2-3631-4'
titleAmericana/title
price16.95/price
/cd
/order
和：books.xml
？xml version="1.0"？
!-- This file represents a fragment of a book store inventory database --
bookstore
book cc="dd" xmlns:bk="urn:sample" xmlns:ns="http://www.Any.com" genre="autobiography" publicationdate="1981" ISBN="1-861003-11-0"
titleThe Autobiography of Benjamin Franklin/title
ns:author
first-nameBenjamin/first-name
last-nameFranklin/last-name
/ns:author
price8.99/price
/book
book genre="novel" publicationdate="1967" ISBN="0-201-63361-2"
titleThe Confidence Man/title
author
first-nameHerman/first-name
last-nameMelville/last-name
/author
price11.99/price
/book
book genre="philosophy" publicationdate="1991" ISBN="1-861001-57-6"
titleThe Gorgias/title
author
namePlato/name
/author
price9.99/price
/book
/bookstore
我们可以对该XML文件中的price求和，以得到价格总数。
Evaluate.cs
//Corelib.net/System.Xml.Xsl/XPathNavigator Class
//Author :Any
using System;
using System.IO;
using System.Xml;
using System.Xml.XPath;
public class EvaluateSample
{
public static void Main()
{
EvaluateSample myEvaluateSample = new EvaluateSample();
myEvaluateSample.test("books.xml");
}
public void test(String args)
{
try
{
//test Evaluate(String);
XPathDocument myXPathDocument = new XPathDocument(args);
XPathNavigator myXPathNavigator = myXPathDocument.CreateNavigator();
Console.WriteLine(myXPathNavigator.Evaluate("sum(descendant::book/price)"));
//testEvaluate(XPathExpression);
XmlDocument doc = new XmlDocument();
doc.Load("order.xml");
XPathNavigator nav = doc.CreateNavigator();
XPathExpression expr = nav.Compile("sum(//price/text())");
Console.WriteLine(nav.Evaluate(expr));
//testEvaluate(XPathExpression);
XPathNodeIterator myXPathNodeIterator = nav.Select("descendant::book/title");
expr = nav.Compile("sum(//price/text())");
Console.WriteLine(nav.Evaluate(expr,myXPathNodeIterator));
}
catch (Exception e)
{
Console.WriteLine ("Exception: {0}", e.ToString());
}
}
}
运行这个程序，结果如下：
30.97
36.9
36.9
我们可以看到，30.97是books.xml中所有price值的总和，而36.9则是order.xml中所有price值的总和。通过XPAH不仅可以快速查找信息，而且还可以对信息进行一些基本的处理。
